The present invention relates generally to EMI/RFI shielding devices, and more specifically, to a self-terminating EMI/RFI shielding gasket and a method of making the same.
Various types of EMI/RFI gaskets are known for reducing the transmission of electromagnetic interference (EMI) and radio frequency interference (RFI). Among the known devices are gaskets having a resilient core surrounded by a deformable wire-mesh gasket material. Suitable known wire-mesh materials are tin-plated phosphor bronze, tin-coated copper-clad steel, silver-plated brass, monel, beryllium copper and aluminum. Different core cross sections and materials are used depending upon the particular application. Wire-mesh gaskets are generally used to shield conventional computer or other electronic equipment by compressing the gasket around an openable access panel, door, or the like.
Known wire-mesh gaskets are manufactured in various lengths and then cut to size for particular installations. In so doing, however, the wire-mesh tends to fray at the terminal ends of the gasket and thus produces an unacceptable appearance. A secondary treatment such as the in situ application of an adhesive has previously been utilized to overcome the poor appearance of the fraying ends. That treatment is a labor intensive process, however, and is not well suited for quick and cost-productive installations.
Other prior art gaskets have attempted to overcome this problem by knitting a conductive layer over a foam core before the foam has completely cured. In such cases, the uncured foam adheres to the knitted layer to prevent the terminal ends from fraying; however, utilizing an uncured core presents problems of dimensional stability and thus limits the shapes and sizes that can be successfully manufactured. U.S. Pat. No. 5,045,635, assigned to Schlegel Corporation, for example, describes a process whereby a urethane foam core expands and cures in a traveling mold that is surrounded by a sheath with a conductive surface having embedded metal fibers or the like.
In addition to aesthetic considerations, EMI/RFI leakage may also occur due to the reduced shielding coverage that occurs as a result of the fraying terminal ends of the gaskets. Further, cutting the wire-mesh gasket during installation may produce particles of metallic debris within the shielded housing.
A further design consideration for EMI/RFI gaskets is galvanic compatibility in the ultimate application. More particularly, when a conductive gasket for excluding electromagnetic interference is placed between two metal flange plates or metalized plastic components, dissimilar metals having dissimilar electrochemical potentials are quite likely to be placed in contact with one another. The use of dissimilar metals or metal coating/plating in metalized plastics is expected because desirable attributes in flange plates, such as strength and rigidity, are not the same as those for gaskets, such as flexibility and maximum electrical conductivity. The difference in the electrochemical potential of the two dissimilar metals is the force that drives the movement of ions therebetween and thus causes galvanic corrosion. Heretofore, knitted gaskets have conventionally used silver plating as the conductive metal therein. While these gaskets function adequately in shielding against electromagnetic and radio frequency interference, the range of materials on which they can be utilized is limited due to the limited galvanic compatibility of the silver plating.